New paradigm in Lens metrology for lithographic scanner: evaluation and exploration

A new paradigm of lens metrology, which is an on-board in-situ interferometer on a scanner, is evaluated. We called this system as Inline PMI and is based on a shearing type interferometer. Wavefront gradient data is measured and used to reconstruct a full high resolution wavefront. The system was evaluated based on short term and long term stabilities, sensitivity towards system parameters, correlation studies with PMI, a resist-based lens metrology tool and lithographic tests to establish accuracy, and model compliance test against lens model prediction. The lens was detuned with Z7-tilt and Z9 offset to extend the dynamic range of the tests. The metrology demonstrated good repeatability, accuracy and stability as well insensitivity toward environmental parameters and good compliance with lens model predictions. In addition, because of the high resolution nature of the inline PMI system high spatial frequency wavefront content can be recovered. With a derived transfer function we can recover approximately up a spatial frequency of 30 to 40 cycles/pupil diameter. This fills the gap in the power spectrum obtained by low order Zernike terms and traditional high frequency flare measurement from techniques such as disappearing pads. Inline PMI may thus enables a more complete analysis of flare in lithography, which is critical to evaluating double exposure techniques as well as bright field masks with widely varying pattern density. Overall, this on-board interferometry shows good technical performance and fast turnaround time, both of which are essential requirement in low k1-imaging in a manufacturing environment.